When an object such as a boat or a plane is rotating or turning, a gyroscope can measure the rate of turning and the angle turned. Such information, when fed into a navigational control, can help ensure that the object maintains the correct heading at all times.
Conventional gyroscope systems include magnetic compasses, rotating wheel or sphere gyroscopes, laser gyroscopes, and fiber-optic gyroscopes. A rotating wheel gyroscope is basically a spinning top which, when properly supported, will maintain its axis of rotation fixed at all times.
A laser gyroscope has two counter-travelling laser beams in an annular ring resonant cavity. Under rotation around an axis perpendicular to the plane of the ring, the optical paths of the two beams differ, causing a corresponding difference in the frequencies of the two beams. The frequency difference is measured to determine the rate of rotation.
A fiber-optic gyroscope has a spool of optical fiber with counter-travelling laser beams of a fixed frequency in the fiber. The two beams emerging from the fiber are interfered with each other and their relative phase difference is measured. The beam paths differ under rotation, causing a phase change to occur. The phase change is proportional to the measured rate of the rotation. A good general discussion of fiber-optic gyroscopes is set forth in the article "Fiber-Optic Gyroscope," IEEE Spectrum, March, 1986, p.54.
Each of the above conventional gyroscopes contains significant limitations. In rotating wheel or sphere gyroscopes, the use of moving parts limits their applications to low-acceleration low dynamic platforms. Laser gyroscopes are expensive and require precision optical parts and fragile optical components. Fiber-optic gyroscopes are potentially low cost, but they are prone to relatively low accuracy due to photon shot noise.
In an attempt to overcome the limitations of conventional gyroscope systems, superconducting gyroscopes have been studied for some years. General discussions of superconducting gyroscopes are contained in the article "A Superconducting Gyroscope With No Moving Parts," IEEE Trans. Magnetics, Vol. 36, p. 170 (1981), and in the article "Superconducting Thin Film Gyroscope Readout For Gravity Probe-B," IEEE Trans. Inst. & Mers., Vol. 36, p. 170 (1987).
From superconducting physics, it is known that if a superconductor element is rotated with an angular velocity of the superconducting electrons on the surface of the superconductor lag behind in the rotation slightly and create a uniform London field B.sub.L in the body of the superconductor. The London field can be expressed by the equation: EQU B.sub.L =2m.omega./q (1 )
where m is the electron mass and q is the absolute value of the electron charge.
Equation (1) indicates that the London field vector B.sub.L is linearly proportional to the rotation vector .omega.. This suggests that from a measurement of the London field, the amount of rotation the superconductor is undergoing synchronously with the moving object can be determined. However, because the London field is very small, it is hard to measure accurately. Additionally, the London field can be easily interfered with by external stray fields.
It is also known that if an infinitely long cylindrical ferromagnetic core is rotated with an angular velocity of .omega., a Barnett field is generated in the core due to the so-called Barnett moment associated with the rotation. The Barnett field can be expressed by the equation: EQU B.sub.B =2m.omega./qg' (2)
where g' is the gyromagnetic ratio of the ferromagnetic material and is typically close to 1.9.
In principle, either of the effects described above can be used in the design of a rotation rate measurement device. The Barnett field and the London field can be measured by a Superconducting Quantum Interference Device ("SQUID"), which is one of the most sensitive magnetic field sensors. In order to make a practical gyroscope, the external fields due to magnetic field sources such as the earth have to be shielded so as not to interfere with B.sub.L and B.sub.B. The earth's magnetic field is normally many orders of magnitude stronger than B.sub.L or B.sub.B.
A typical example of such measurement devices is suggested by the article "Inertial and Gravitational Experiments with Superfluids: A Progress Report," Proceedings of the 4th Marcel Grossman Meeting on General Relativity, Elserier Science Publishers B.V., #1312 (1986). This device includes a long ferromagnetic cylindrical core for providing a Barnett field, a superconducting pick-up coil surrounding the long magnetic core for picking up the Barnett field and the London field, a SQUID which is magnetically coupled to an input coil connected to the pick-up coil for measuring the sum of the Barnett field and the London field due to the rotation of the system, and a superconducting shield around the entire device for preventing the measurement system from being interfered with by outside signals.
This device, however, has several problems which makes it unsuitable for practical use. The long magnetic rod occupies a large space. In addition, there is always some minute trapped flux inside the superconducting shield. The trapped flux will induce spurious signals known as microphonic noises in the pick-up coil when there are vibrations between the superconducting shield and the magnetic core. Furthermore, the use of a pick-up coil and an input coil will reduce detection efficiency due to the energy loss in the dual-coil signal conversion process. The noise problem and energy loss result in a high minimum detectable rotation rate and low measurement signal-to-noise ratio. In summary, such devices are undesirable because they have a large size, high noise level and resultant poor accuracy.